This invention relates to trimmable circuits and more particularly to an integrated circuit which is trimmed by electrical programming signals that are used to modify the values of selected components of the integrated circuit by shorting zener diode fuse structures.
Zener diodes are widely used in the integrated circuit art as programmable fuses that can be "blown" or converted from an essentially open circuit condition to an essentially short circuit condition by the application of a high current pulse to the zener fuse structure. One particularly important application involves using zener fuses as programmable elements that are used to modify the values of circuit components so that the performance parameters of a circuit function can be adjusted to fall within precise limits. In this application, the shorting mechanism of the zener fuse is used to modify the operation of a circuit by either shorting out a component such as a resistor, capacitor or transistor, or by completing the connection of one or more partially connected components.
To date, the use of zener trimming techniques has been limited to either high voltage circuits, or to circuits that have been electrically and physically designed to withstand the very high supply voltages of 10 to 25 volts that have been needed to program zener diode fuses. Further, most of the trim networks that have been designed to effect component value changes via the programming of zener fuses will not operate over the low supply voltages range of 1.0 to 1.5 volts that is of importance in many battery operated applications including radio paging receivers.
The restrictions imposed by the present zener trimming techniques thus mean that the techniques either cannot be directly applied to low voltage circuits, or that they negatively impact the cost of these circuits. The higher cost factor is a direct consequence of the present need for zener trimmed circuits to withstand the high supply voltage associated with the trimming procedure. Since low voltage circuits are normally designed with closer physical layout spacings than high voltage circuits, the use of high voltage layout spacings results in larger integrated circuit chip sizes and higher costs.